When I looked closely at the center of the pictures (at least in the original unscaled versions), I could see concentric rings. When I first saw them, I found them to be quite strange, but interesting nevertheless.
At first, I thought they were Airy rings, but after thinking for a while, I realized that there is no small aperture from which they could arise. After some more thought, I found the solution - they are interference rings from somewhere in the optics. My suspicion fell on the UV filter I bought for my lens, a suspicion which was confirmed once I photographed another aurora without the filter.
So, where does this interference pattern come from, and what can we learn from it?
The first thing to note, is that the fact that there is an interference pattern, implies that the light is monochromatic, or at least coming from a very narrow wave band. Since I can count at least 30 rings, the waveband should be no more than about 1/30 of the wavelength. Since green is about 5500Å, the emitted waveband has to be narrower than about 200Å. In fact, the radiation is monochromatic. It comes from a well known 5577Å forbidden line of Oxygen, namely, under lab conditions, they cannot be observed.